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The UV- and heat-induced cis→trans isomerization of undoped polyacetylene (I) [25067-58-7] films was investigated. Temp. and UV light promote the isomerization with a similar mechanism. The formation of paramagnetic defects in trans I takes place by bond rehybridization, in agreement with previous hypotheses, and both spin concn. and spin delocalization depend on the temp. of isomerization. Under UV irradn. thermal cis→trans isomerization of I also takes place at temps. at which the sole thermal treatment isomerization is much lower or nil, so it may be possible to prep. trans-I under relatively mild conditions, leading to a better material